Mackie M 1200 Manuel du propriétaire télécharger pdf (Page 18)

THRU

Someday you’ll do a show at Carnegie Hall

and realize that one M•1200 or M•1400 ampli-

fier is just not going to do the job — you’ll need

a six-foot-high rack full of ’em. That’s what the

THRU

jack is for. Simply plug the signal source

outputs into the first amp’s

INPUT

s , patch

from that amp’s

THRU

jacks to the next amp’s

INPUT

s, and so on, daisy-chaining as many

amps as you can afford (assuming your console

has low-impedance outputs).

A general rule of thumb is to

maintain a load impedance

10 times or more than the

source impedance to prevent

excessive loading. If your

console has an output impedance of 100 ohms,

then you can daisy-chain up to twenty M•1200/

M•1400 amplifiers, which presents a load of

1000 ohms to the console (input impedance of

20 kohms divided by 20 amplifiers = 1000 ohms).

The

THRU

jacks can also be used to relay

the signal on to other devices such as a DAT or

cassette recorder, enabling you to record ex-

actly what the audience is hearing. The

THRU

jacks are wired straight from the XLR and TRS

INPUT

s — there is no electronic circuitry

between — so the signal going into the amp is

exactly the same as the signal coming out of

the

THRU

jacks.

You can use the

THRU

jack

as an input, if necessary,

since it’s wired in parallel

with the other input connec-

tors. You can also use the

1/4" TRS

INPUT

jacks as

THRU

jacks. Simply

connect the 1/4" TRS

INPUT

jacks on the first

amplifier to the TRS 1/4"

INPUT

jacks on the

second amplifier using 3-conductor shielded

cables with TRS plugs on both ends.

Warning: If you use a regular guitar cord with

2-conductor TS plugs, you’ll unbalance the sig-

nal at the XLR input by grounding the low side

(–) of the signal (pin 3).

LOW CUT FILTER

Every woofer has frequency response speci-

fications. It’s usually expressed in Hertz (or

cycles per second), like “40Hz–300Hz.” The

“40Hz” refers to the low-frequency point (usu-

ally, but not always) where the speaker’s

output drops by 3 dB, and will "roll off" com-

pletely as the frequency goes down. There is no

point in sending a woofer any frequencies it

can’t reproduce — you can’t hear it, and worse

yet, it’s a waste of amplifier power that can be

better used reproducing frequencies you can hear.

100

10k

20k

–15

–10

–5

In order to match the output bandwidth

with your particular speaker system, the

M•1200/M•1400 amplifier has a tunable

LOW

CUT FILTER

. The frequencies are clearly

marked along the knob’s travel

• Fully counterclockwise, the frequency is

below 10Hz, effectively bypassing the filter.

• Center detent is

35Hz

and labeled

TYPICAL

since precious few woofers actually go

below that.

• 3/4 of the way up is labeled

STAGE MONI-

TOR, 100Hz

, perfect for, well, stage monitors

(they seldom reproduce below 100Hz;

besides, it prevents low-frequency “leak-

age” into the house).

• Fully clockwise is labeled

170Hz

So, grab your woofer’s spec sheet and find

the low roll-off spec. Then set the M•1200/

M•1400’s

LOW CUT FILTER

at the same

frequency.

If you do this correctly (and make sure the

meters

are happy), you’ll never again see

your woofer moving sporadically without au-

dible signal. Your system will play louder and

cleaner, and you may never blow another

woofer again!

CONSTANT DIRECTIVITY

Compression drivers mounted on constant-

directivity horns require compensation, in the

form of a high-frequency boost, with its “knee”

set somewhere between 2.5kHz and 6kHz.

Until now, you’d have to resort to external

crossovers or worse yet, graphic EQ modules.

Both of these are fraught with limitations, not

to mention adding cost, rack space require-

ments and complexity to your system.

The M•1200/M•1400 Power Amplifiers

eliminate the need for any of these external

devices — they have the compensation cir-

cuitry already built in. And using it is a breeze.

Low Cut Filter Frequency Response

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Mackie M 1200 Manuel du propriétaire Page 18

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